1. Field of the Invention
This invention relates to electro-acoustic apparatus, particularly to surface acoustic wave devices.
2. Description of the Prior Art
Surface acoustic wave devices normally utilize a transducer suitable for propagating surface acoustic waves or Rayleigh waves with a transducer at one end of a substrate for launching surface acoustic waves and a transducer position at the other end of the substrate for receiving surface acoustic waves. The transducers convert electrical energy into acoustic wave energy and vice versa. The concomitant use of substrate material for propagation of surface acoustic waves has been suggested in U.S. Pat. No. 3,582,540 issued on June 1, 1971 to Robert Adler where transducers have been positioned on the four sides of a rectangular substrate. Two transducers on two adjacent sides of a substrate launch surface acoustic waves through the central portion of the substrate to a transducer on the opposite side as shown in FIG. 18. The patent states at column 17 that more efficient use of the substrate may be obtained by having different surface acoustic waves crossing one another utilizing a common area for propagation.
The common use of substrate area by interdigital fingers of a three-phase transducer is described by C. S. Hartmann, et al. entitled "Wide-Band Unidirectional Interdigital Surface Wave Transducers" appearing in IEEE Transactions on Sonics and Ultrasonics, Vol. S.U.-19, No. 3, pp. 378-381, July 1972. A three-phase transducer is shown in FIG. 1 comprising three sets of fingers evenly spaced apart and positioned adjacent respective fingers of the other sets. The fingers are interconnected to form three electrode groups which are electrically driven 120 degrees out of phase.
The use of a plurality of transducers on a substrate including switches for selectively coupling a number of transducers to an input or output circuit is described in U.S. Pat. No. 3,831,116 issued on Aug. 20, 1974 to L. Davis, Jr. and M. G. Holland. As shown in FIG. 3 a plurality of spaced-apart transducers are positioned on a substrate. Each transducer is coupled through a switch to either an input or output circuit. The switches may be set as desired to produce a desired arrangement of frequency response peaks from the input to the output. The switchable filter may be used, for example, in a receiver where the switches are set by an operator on the switchable filter to select a desired frequency.
Surface acoustic wave devices have also been fabricated which utilize the propagation time delay of the substrate and frequency response of the taps to form an oscillator having predetermined modes of oscillation dependent upon the time delay of a signal traveling through the device through a feedback amplifier to the input of the surface acoustic wave device. An example of an oscillator incorporating a surface acoustic wave device is shown in U.S. Pat. No. 3,855,548 issuing on Dec. 17, 1974 to A. K. Nandi, et al. A surface acoustic wave device is shown having an input transducer, a propagation path for surface acoustic waves in the substrate and an output transducer having a plurality of output taps coupled together to an output terminal. The plurality of output taps each have an internal structure comprising a few pairs of interdigitated electrodes. The plurality of output taps function to provide a frequency response to provide selected feedback to the input of a surface acoustic wave device. In other words, the output signal as recirculated back through the surface acoustic wave device which will oscillate at preferred frequencies dependent upon the recirculating time delay.
In the prior art, a number of filters utilizing surface acoustic wave devices would each utilize a separate surface acoustic wave substrate area for its filter. For example, to implement a selectable frequency band-pass filter, a number of surface acoustic wave filter devices were shown each on a separate substrate which was described in U.S. Pat. No. 3,855,556 issuing on Dec. 17, 1974. A particular frequency was selected by switching in a filter responsive to the desired frequency.
It is, therefore, desirable to provide for simultaneous use of the same area on a surface acoustic wave substrate for several filters.
It is further desirable that several filters may be fabricated on a surface acoustic wave device for occupying the same area.
It is further desirable that several comb filters be fabricated on a surface acoustic wave device for occupying the same area, each operating independently of the others and having separate concurrent outputs.
It is further desirable to provide a plurality of frequencies, each on a separate output line, selected from an input signal having a comb frequency spectrum.
It is further desirable to provide signal generation means for rapidly switching between frequencies within a predetermined bandwidth determined by a comb frequency spectrum derived from a crystal-controlled oscillator.
It is further desirable to fabricate a surface acoustic wave device which allows for the coexistence of a bank of transducers on the same surface acoustic surface area for selection of individual lines from a comb spectrum and sustaining oscillations of the comb spectrum by appropriate feedback.